The present invention relates to an interference wave detection circuit for detecting whether a beat due to the difference between two carrier frequencies is included or not in a received signal of a radio receiver, and more particularly to an interference wave detection circuit suitable for use in a mobile radio communication system of multi-frequency access type without provision of a central control station.
Mobile radio communication systems of this kind have an application to a cordless telephone system. As illustrated in FIG. 1, radio concentrators A, B and C are connected to respective subscriber lines SL.sub.A, SL.sub.B and SL.sub.C and are individually coupled to corresponding radio telephone sets A', B' and C' via radio channels, respectively. If different frequencies are allocated to respective radio channels, there will be no interference among these channel under normal condition. However, such an individual frequency access system is not preferable in view of effective utilization of frequencies. To improve this, it is necessary to utilize a small number of frequencies assigned in common to different sets of radio telephone equipment.
It is now assumed that the cordless telephone system is configured so that two sets of transmission/reception frequencies f.sub.1, f.sub.1 ' and f.sub.2, f.sub.2 ' are assigned to three sets of radio telephone equipment comprising radio concentrators and radio telephone sets and all radio equipment can utilize the two sets of frequencies. When the frequencies f.sub.1, f.sub.1 ' are used by the radio concentrator A and the radio telephone set A', the radio concentrator B and the radio telephone set B' are assumed to start utilization of the same set of frequencies. In such a case, when the telephone system recognizes or acknowledges that the frequencies f.sub.1, f.sub.1 ' are in use, it automatically switches the used frequencies f.sub.1, f.sub.1 ' to the other set of frequencies f.sub.2, f.sub.2 ' to establish a radio channel for the radio concentrator B and the radio telephone set B'.
In this instance, when the radio concentrator C and the radio telephone set C' are to start the use of the channel at the same time, it is detected that both the sets of frequencies f.sub.1 and f.sub.1 ', and f.sub.2 and f.sub.2 ' have been already used. As a result, the channel connection between the concentrator C and the telephone set C' becomes impossible, and therefore they must wait until the frequency occupation by either the radio telephone equipment A, A' or B, B' is released.
In the case where a large number of the above-mentioned systems are installed as terminal equipment for a public communication network, there is a possibility that a radio concentrator D and a radio telephone set D' belonging to another group of telephone equipment attempt to establish a radio channel by the use of frequencies f.sub.1, f.sub.1 '. When this telephone equipment is located far from the first-mentioned group of the telephone equipment to the extent that it does not cause interference therewith, any problem does not arise. However, if the radio telephone set A' moves closer to the radio concentrator D, there may occur interference therebetween. In such a case, it is preferable that the radio telephone sytem be configured so that the radio concentrator D automatically detects the interference and switches to another empty or idle frequency set. To realize this, a system for automatically and precisely detecting the occurrence of interference is required.